Adolescent idiopathic scoliosis (AIS) is the most common pediatric spinal deformity. To develop personalized prevention strategies for those at high risk and to eliminate screening and treatment of those at low risk of progression, accurate methods of predicting curve progression are needed. Unfortunately, currently available algorithms of curve progression for AIS are inaccurate, possibly because the role of rare genetic factors has been largely unexplored. By focusing on extreme cases with severe scoliosis (curve >40), we recently completed a genome-wide rare variant burden analysis of exome sequence data of 91 cases and validated a significant enrichment of rare variants in FBN1 (fibrillin-1) and FBN2 (fibrillin-2) in AIS cases compared to controls. Furthermore, evidence of TGF- pathway activation was seen in paraspinous muscle of AIS patients with rare FBN1 variants, suggesting the possibility of novel treatments for AIS. However, additional studies are needed to identify the full complement of genetic risk factors for severe scoliosis, and to confirm a role for the TGF- pathway in its pathogenesis. The goal of this project is to identify genetic risk factors for sever AIS. Here, we propose a multicenter genome-wide rare variant association study of AIS, focusing on extreme cases of severe scoliosis that are likely enriched for genetic factors of large effect. This study utilizes DNA samples collected from five Bracing in Adolescent Idiopathic Scoliosis Trial (BrAIST) sites, and three new sites. A genome-wide rare variant association study will be performed on exome sequence data from 1000 severe AIS cases, followed by validation of the most highly associated variants/genes in AIS cases of both European and Han Chinese ancestry. Functional studies of AIS-associated rare variants will be evaluated using patient biobanked tissue, in vitro assessment of TGF- pathway activation, and generation of zebrafish mutants. Knowledge of the genetic risk factors for severe scoliosis is essential for accurate prediction of curve progression and development of new treatment strategies.